1. Field of the invention
A field of the invention is that of aircraft gas turbine engine rotor blades. The invention relates to a method for machining rotor blade wear indicators.
2. Description of The Related Art
A turbine disk of a gas turbine engine conventionally comprises a plurality of turbine blades secured to the disk and driven in rotation by a flow of hot air from the combustion chamber of the engine. As the engine ages, functional clearances arise and the turbine blades may come into contact with the engine casing and this wears the blade ends.
SNECMA application FR 2 928 405 discloses a turbine blade which, at its distal end comprises an open cavity or bathtub squealer, delimited by a bottom wall which extends over the entire tip of the blade and by a circumferential side wall that forms a rim. The rim provides a wearing region between the bottom wall and the casing that allows contact between the distal ends of the blade and the casing to be absorbed.
In order to measure blade wear, it is known practice to machine wear indicators into the ends of the blades when the blades are new. Conventionally, the wear indicators are formed on the suction face of the blade and take the form, firstly, of a series of radial grooves formed in the rim of the blade bathtub squealer and secondly of a chamfer formed near the trailing edge of the blade.
Thus, when the blade comes into contact with the engine casing, the rim of the bathtub squealer becomes worn, together with its wear indicators. In order to determine the degree of blade wear, a visual inspection is carried out to check whether the wear indicators are still visible or whether their appearance has changed. For preference, the radial grooves are of increasing length and correspond respectively to increasing levels of wear.
In order to form the wear indicators it is known practice for the blade to be machined using an EDM (Electrical Discharge Machining) method or by milling. The machined blade is then coated with a thermal barrier coating known by the abbreviation TBC. To form the turbine rotor, a plurality of coated blades is mounted on a turbine disk. Conventionally, for a turbine rotor, of the plurality of blades just a few comprise wear indicators. These few blades are angularly distributed at the periphery of the turbine disk.
In order for the turbine rotor to be concentric, a surface grinding step in which all the turbine rotor blades are machined at their ends is conventionally carried out in order to correct turbine rotor concentricity defects. The result of this is that a new turbine rotor, i.e. one that has come directly from the manufacturing stage, may exhibit blades the wear indicators of which have been machined because of the surface grinding operation. In other words, a wear indicator may indicate that a blade is worn even though this blade is new.
In order to eliminate this disadvantage, it has been proposed for the wear indicators to be machined again after surface grinding so that these indicators correspond to those of a new blade. One immediate solution would be to machine the surface-ground blades that have a thermal barrier coating TBC using an EDM method or by milling, but this would impair the thermal barrier coating. In particular, milling coated blades causes the thermal barrier coating to “flake” and lose its effectiveness. The other disadvantages of this type of machining are that it carries the risk of contaminating the surface of the coated rotor blades and that it is awkward and expensive to implement.
Another solution would be to machine the blades individually but that would entail removing the blades from their turbine disk beforehand and this would considerably lengthen the machining method and increase the cost thereof. By way of example, a method according to the prior art requires at least one day of manual work in order to obtain a satisfactory result.